Fat Black Holes Can Lurk in Thin Galaxies

Supermassive black holes are thought to lurk at the heart of most galaxies. Scientists have long believed that only the galaxies with thick central bulges could pull together enough mass for a supermassive black hole to form. But NASA’s Spitzer Space Telescope has turned up evidence that even skinny galaxies, with no central bulge, can still form these galactic monsters.
Astronomers used the Spitzer Space Telescope to survey 32 flat and bulgeless galaxies, and still turned up supermassive black holes in their central cores. This means that galaxy bulges aren’t necessary to build up these black holes; instead, the mysterious and invisible dark matter might be necessary to bring them together.

“This finding challenges the current paradigm. The fact that galaxies without bulges have black holes means that the bulges cannot be the determining factor, said Shobita Satyapal of George Mason University, presenting her research at the American Astronomical Society’s Winter meeting in Austin. “It’s possible that the dark matter that fills the halos around galaxies plays an important role in the early development of supermassive black holes.”

Seen from edge on, our own Milky Way’s bulge would be clearly visible, with the thin spiral arms trailing away to the sides. And researchers know we have a supermassive black hole. Researchers used to think there was a direct connection between the size of the bulge, and the mass of the black hole.

But in 2003, astronomers discovered a relatively lightweight black hole in a galaxy without a bulge. And then earlier this year, Satyapal and her team found another example of this bulgeless black hole.

Since bulges don’t seem to matter, Satyapal suggests that a galaxy’s dark matter halo is the deciding factor to determine how massive a black hole can get.

“Maybe the bulge was just serving as a proxy for the dark matter mass – the real determining factor behind the existence and mass of a black hole in a galaxy’s center,” said Satyapal.